Data acquisition unit, system and method for geophysical data

ABSTRACT

A data acquisition system for gathering geophysical data, a corresponding method, and a data acquisition unit for use with the system and method are disclosed. The system ( 10 ) comprises a plurality of data acquisition units ( 14 ) for gathering geophysical data, each data acquisition unit ( 14 ) being connectable to at least one sensor ( 15 ) and being arranged, during use, to gather geophysical data from the at least one sensor ( 15 ). Each data acquisition unit ( 14 ) comprises time referencing means ( 48 ) arranged to generate time reference data usable to control the time at which samples of geophysical data are taken. The system ( 10 ) further comprises means for calculating spatial derivatives between samples associated with adjacent sensors ( 15 ) connected during use to the data acquisition units ( 14 ).

FIELD OF THE INVENTION

The present invention relates to a data acquisition unit, system andmethod for geophysical data and, in particular, to such a dataacquisition unit, system and method for use in geophysical surveysarranged to measure electric and/or magnetic fields and generate surveydata on the basis of the measured field.

BACKGROUND OF THE INVENTION

It is known to provide a data acquisition system which includes aplurality of networked data acquisition units, each data acquisitionunit being connected to at least one sensor and being arranged to gathersurvey data from the sensors. The received survey data is passed via thenetwork to a central computing device for processing. Synchronisation ofthe received survey data is also carried out via the network.

However, a disadvantage of this arrangement is that the gathered surveydata often includes a significant amount of noise which can be ofsufficiently large magnitude to obscure the desired signal responseassociated with a relatively deeply buried target.

In the claims which follow and in the preceding description of theinvention, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a data acquisition system for gathering geophysical data, saidsystem comprising:

at least one data acquisition unit connectable to a plurality of sensorsand being arranged, during use, to simultaneously gather geophysicaldata from the sensors, the or each data acquisition unit comprising timereferencing means arranged to generate time reference data usable tocontrol the time at which samples of geophysical data are taken; and

-   -   means for calculating spatial derivatives between simultaneous        samples associated with adjacent sensors connected during use to        the at least one data acquisition unit so as to provide        processed geophysical data with less noise.

Preferably, the time referencing means includes a GPS receiver.Alternatively or in addition, the time referencing means may include anaccurate oscillator, preferably a precision oven controlled crystaloscillator, and a counter arranged to count signals generated by theoscillator.

In embodiments which include an oscillator, the data acquisition unit ispreferably arranged to receive synchronisation signals useable by theprocessing means to adjust the frequency of the oscillator and adjustthe times at which samples of geophysical data are taken so that thetimes at which samples of geophysical data are taken are synchronisedwith the times at which samples of geophysical data are taken in otherdata acquisition units.

Preferably, the data acquisition unit is arranged to receive programsand to store the programs in the data storage means for subsequentexecution by the processing means.

Preferably, the data acquisition unit is arranged to calculate anaverage sample value for a plurality of corresponding repeat samplevalues when a plurality of data gathering operations are carried out aspart of a geophysical survey so as to reduce the effect of interferenceon the samples and reduce the quantity of data. The data acquisitionunit may be arranged to compare repeat samples and to discard sampleswhich differ by a predetermined amount from the majority of the repeatsamples.

Preferably, the data acquisition unit is arranged to calculate anaverage sample value for a plurality of consecutive samples taken duringa data gathering operation carried out as part of a geophysical surveyso as to produce a representative sample for the consecutive samples.

Preferably, the data acquisition unit is arranged to estimate the amountof interference present at a survey site. The amount of interferencepresent may be estimated by carrying out a first data gatheringoperation with an incident magnetic field of a first polarity so as toproduce a first response, carrying out a second data gathering operationwith an incident magnetic field of a second polarity so as to produce asecond response, and calculating the sum of the first and secondresponses so as to cause the first and second responses to cancel out.

Preferably, the data acquisition unit is arranged to filter gatheredgeophysical data so as to remove periodic interference.

Preferably, the data acquisition unit is arranged to convert gatheredgeophysical data into frequency domain using Fourier transform analysis.

Preferably, the data acquisition unit is arranged to generate a leastone quality control indicator for use in assessing the quality of thegathered geophysical survey data.

Preferably, the data acquisition unit is arranged to calculate astandard deviation value for the gathered geophysical survey data.

Preferably, the data acquisition unit is arranged to adjust the level ofgain applied to gathered geophysical survey data based on an assessmentof the magnitude of the gathered geophysical survey data.

Preferably, the system is arranged to downward extrapolate gatheredgeophysical survey data so as to enhance detail of a target locatedbelow the surface of a survey area.

Preferably, the data acquisition unit is connectable to an energy sourceand the data acquisition unit is operable as an energy source controlunit.

Preferably, the data acquisition unit includes at least one interfacearranged to facilitate transfer of processed geophysical data and/orprograms to or from the data acquisition unit. For this purpose, theinterface may include an infra red interface, a serial interface and/ora network interface. The interface may be of a type which utiliseswireless protocols, such as Bluetooth.

In embodiments which include an oscillator, a synchronisation interfacemay be provided for facilitating transfer of synchronisation signals toand/or from the data acquisition unit for the purpose of ensuringcorrect synchronisation of the oscillator with oscillators of other dataacquisition units.

Preferably, the data acquisition unit includes display means, which maybe an LCD display and/or an LED display, arranged to provide informationto an operator as to whether operation of the data acquisition unit issatisfactory and/or whether the processed survey data is of sufficientquality for subsequent analysis. Such information may indicate whetherthere is a fault with the data acquisition unit or with a sensorconnected to the data acquisition unit, or whether other conditionsexist which necessitate operator action.

Preferably, the data acquisition unit is arranged to store a correctioncoefficient for each sensor connected during use to the data acquisitionunit, each correction coefficient being used to correct for variationsin sensor sensitivity.

Preferably, the data storage means is a FLASH memory. Additionally, ahard disk drive may be provided.

Preferably, the system includes a plurality of data acquisition units.

In one arrangement, the data acquisition unit includes the means forcalculating spatial derivatives.

In an alternative arrangement, the means for calculating spatialderivatives is separate to the data acquisition unit. With thisarrangement, the system may further comprise a portable computingdevice, the portable computing device including the means forcalculating spatial derivatives.

Preferably, the system further comprises at least one reference dataacquisition unit, each reference data acquisition unit being connectableto at least one reference sensor and being arranged, during use, togather geophysical data from the at least one reference sensor, and totake samples of the geophysical data gathered from the sensors; whereinthe means for calculating spatial derivatives between samples associatedwith adjacent sensors is arranged to calculate first spatial derivativesbetween at least some of the sensors connected to the data acquisitionunits and a reference sensor connected to the reference data acquisitionunit during a first data gathering operation when the sensors aredisposed in a first location, to calculate second spatial derivativesbetween at least some of the sensors connected to the data acquisitionunits and a reference sensor connected to the reference data acquisitionunit during a second data gathering operation when the sensors aredisposed in a second location, and to calculate a difference spatialderivative between the first and second spatial derivatives, each saiddifference spatial derivative being indicative of a spatial derivativebetween a sensor disposed in a first location and a sensor disposed in asecond location.

Preferably, the system further comprises means for calculating anintegral of the spatial derivatives so as to produce conventionalgeophysical data with less noise.

In one arrangement, the system also includes an energy source arrangedto generate and direct energy towards the sub-surface volume so as tocause a geophysical response and thereby cause generation of thegeophysical signals.

Preferably, the energy source includes a transmitter and a transmitterloop.

Preferably, the system is arranged to correct for variations inmagnitude of the transmitter current during a geophysical survey. Thesystem may be arranged to correct for a variation in magnitude of thetransmitter current caused by a variation in power supplied totransmitter.

Preferably, the system also includes an energy source control unitconnectable to the energy source and arranged to gather output data fromthe energy source, the energy source control unit including:

time referencing means arranged to generate time reference data usableto control the time at which gathering of the energy source output dataoccurs and to associate the energy source output data with the timereference data; and

data storage means for storing the energy source output data.

Preferably, the energy source control unit is a transmitter control unitarranged to control a transmitter so as to energise a transmitter loopin accordance with a predetermined frequency.

Preferably, the energy source control unit includes the same componentsas the data acquisition unit so that the transmitter control unit iscapable of carrying out the functions of the data acquisition unit andvice versa.

Preferably, the system is arranged to correct for variations inmagnitude of the transmitter current during a geophysical survey. Thesystem may be arranged to correct for a variation in magnitude of thetransmitter current caused by a reduction in power supplied to thetransmitter.

In accordance with a second aspect of the present invention, there isprovided a method of acquiring geophysical data, said method includingthe steps of:

providing at least one data acquisition unit arranged to simultaneouslygather geophysical data from a plurality of sensors connected in use tothe at least one data acquisition unit;

connecting at least one geophysical sensor to the at least one dataacquisition unit;

generating at the data acquisition unit time reference data usable tocontrol the time at which gathering of samples of geophysical data aretaken; and

calculating spatial derivatives between simultaneous samples associatedwith adjacent sensors connected during use to the at least one dataacquisition unit so as to produce processed geophysical data with lessnoise.

Preferably, the method further comprises the steps of:

providing at least one reference data acquisition unit arranged, duringuse, to gather geophysical data from the at least one reference sensor;

connecting each reference data acquisition unit to at least onereference sensor;

calculating first spatial derivatives between at least some of thesensors connected to the data acquisition units and a reference sensorconnected to the reference data acquisition unit during a first datagathering operation when the sensors are disposed in a first location;

calculating second spatial derivatives between at least some of thesensors connected to the data acquisition units and a reference sensorconnected to the reference data acquisition unit during a second datagathering operation when the sensors are disposed in a second location;and

calculating a difference spatial derivative between the first and secondspatial derivatives, each said difference spatial derivative beingindicative of a spatial derivative between a sensor disposed in a firstlocation and a sensor disposed in a second location.

Preferably, the method further comprises means for calculating anintegral of the spatial derivatives so as to produce conventionalgeophysical data with less noise.

Preferably, the method further includes the step of correctingvariations in the energy source using the reference data acquisitionunit and associated reference sensor.

Preferably, the time referencing means includes a GPS receiver.Alternatively or in addition, the time referencing means may include anoscillator, preferably, a precision oven controlled crystal oscillator,and a counter arranged to count signals generated by oscillator.

In embodiments which include an oscillator, the method preferablyincludes the step of receiving at the data acquisition unitsynchronisation signals useable by the processing means to adjust thefrequency of the oscillator and thereby adjust the time at whichgathering of geophysical data occurs so that the time at which gatheringof geophysical data occurs is synchronised with the time at whichgathering of geophysical data occurs in other data acquisition units.

Preferably, the method further comprises the step of calculating anaverage sample value for a plurality of corresponding repeat samplevalues when a plurality of data gathering operations are carried out aspart of a geophysical survey so as to reduce the effect of interferenceon the samples and reduce the quantity of data.

Preferably, the method further comprises the step of comparing repeatsample values and discarding samples which differ by a predeterminedamount from the majority of the repeat sample values.

Preferably, the method further comprising the step of calculating anaverage sample value for a plurality of consecutive samples taken duringa data gathering operation carried out as part of a geophysical surveyso as to produce a representative sample for the consecutive samples.

Preferably, the method further comprises the step of estimating theamount of interference present at a survey site. The amount ofinterference present may be estimated by carrying out a first datagathering operation with an incident magnetic field of a first polarityso as to produce a first response, carrying out a second data gatheringoperation with an incident magnetic field of a second polarity so as toproduce a second response, and calculating the sum of the first andsecond responses so as to cause the first and second responses to cancelout.

Preferably, the method further comprises the step of filtering gatheredgeophysical data so as to remove periodic interference.

Preferably, the method further comprises the step of converting gatheredgeophysical data into frequency domain using Fourier transform analysis.

Preferably, the method further comprises the step of correcting forvariations in magnitude of the energy source during a geophysicalsurvey. The step of correcting for variations in magnitude may includethe step of correcting for a variation in magnitude of the energy sourcecaused by a variation in power supplied to the energy source.

Preferably, the method further comprises the step of generating a leastone quality control indicator for use in assessing the quality of thegathered geophysical survey data.

Preferably, the method further comprises the step of calculating astandard deviation value for the gathered geophysical survey data.

Preferably, the method further comprises the step of adjusting the levelof gain applied to gathered geophysical survey data based on anassessment of the magnitude of the gathered geophysical survey data.

Preferably, the method further comprises the step of downwardextrapolating gathered geophysical survey data so as to enhance detailof a target located below the surface of a survey area.

Preferably, the method further includes the step of facilitatingtransfer of processed geophysical data and/or programs to or from thedata acquisition unit. For this purpose, the interface may include aninfra red interface, a serial interface and/or a network interface.

In embodiments which include an oscillator, the method may also includethe step of facilitating transfer of synchronisation signals to and/orfrom the data acquisition unit for the purpose of ensuring correctsynchronisation of the oscillator with oscillators of other dataacquisition units.

Preferably, the method further includes the step of providing displaymeans for providing information to an operator as to whether operationof the data acquisition unit is satisfactory and/or whether thetime-stamped processed survey data is of sufficient quality forsubsequent analysis. Said information may indicate whether there is afault with the data acquisition unit or with a sensor connected to thedata acquisition unit, or whether other conditions exist whichnecessitate operator action.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only,with reference to the accompanying drawings, in which;

FIG. 1 is a block diagram of a data acquisition system in accordancewith an embodiment of the present invention;

FIGS. 2 a and 2 b are plots of modelled fixed-loop TEM responses of atarget for X and Z components respectively,

FIG. 3 is a block diagram of a data acquisition unit in accordance withan embodiment of the present invention;

FIG. 4 is a diagrammatic representation of an array of data acquisitionunits deployed in a survey area during use;

FIGS. 5 a and 5 b are diagrammatic representations illustrating a methodof using a relatively small number of data acquisition units to carryout a survey over a relatively large survey area;

FIG. 6 is a block diagram of an interface unit of the data acquisitionunit shown in FIG. 3; and

FIG. 7 is a block diagram of a data acquisition system in accordancewith an alternative embodiment of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

Referring to FIG. 1 of the drawings, there is shown a data acquisitionsystem 10 for gathering geophysical data during a geophysical survey. Inthis example, the system 10 is a TEM (transient electromagnetics) typesystem arranged to generate and sense magnetic fields, although it willbe understood that the invention is equally applicable to othergeophysical surveys, such as geophysical surveys based on electricfields or seismic measurements, including MT (magneto-telluric) and IP(induced polarisation) type surveys.

An example of responses obtained from a modelled fixed-loop TEM typesurvey for a conductive target disposed at a depth of the order of 500 mis shown in FIGS. 2 a and 2 b, where FIG. 2 a shows a profile for an Xcomponent of a response and FIG. 2 b shows a profile of a Z component ofthe response. The responses are displayed in units of microvolts pertransmitter amp for coil sensors of effective area 10,000 sqm. At delaytimes greater than 250 mS, that is 250 mS after deactivation of thetransmitter current, the target becomes identifiable as a polarityreversal 19 in the Z component and a peak 21 in the X component at alocation approximately above the target. As can be seen, a target is notidentifiable until the responses have decayed to around 10 nV/A.Accordingly, in order to detect such a target at a depth of the order of500 m using a TEM type survey, the noise level associated with thesystem must be significantly less than 10 nV/A so that the response isnot obscured by noise.

The system 10 includes a source of energy, in this example in the formof a pair of transmitter loops 12, each transmitter loop 12 generating amagnetic field when an electrical current passes through the loop 12.The magnetic field generated by a transmitter loop 12 passes into theearth's sub-surface and induces currents in electrically conductivecomponents in the sub-surface which in turn generate electro-magneticfields. The transmitter current is turned off after a predeterminedperiod of time which causes the electromagnetic fields to decay inmagnitude over time. The decaying electromagnetic fields are sensed by aplurality of data acquisition units 14 and associated sensors 15disposed around the desired area 17 to be surveyed, and the sensedsurvey data is then sampled at a predetermined sampling rate so as toproduce for each sensor samples of electromagnetic field data whichdecrease in magnitude with each successive sample. The data samples arestored at the data acquisition unit 14. By analysing the data samplesreceived at the data acquisition units 14, it is possible to obtain anindication as to the characteristics of the desired sub-surface volume.In practice, the transmitter loops 12 are energised one at a time andthe response data from both transmitter loops is analysed to provide anindication as to the characteristics of the desired sub-surface volume.

In this example, the sensors 15 are coil-type sensors configured so asto measure a Z component, that is a generally vertical component, of anelectromagnetic field. Each coil has a passive coil area of the order of350 sqm. It is possible to construct such a coil which has relativelylow noise characteristics at frequencies of the order of 10 Hz.

The system 10 also includes an energy source control unit, in thisexample a transmitter control unit 16, and a transmitter 18, thetransmitter control unit 16 being arranged to control the transmitter 18so as to energise the transmitter loops 12 in accordance with apredetermined frequency. The transmitter control unit 16 also serves tosample the transmitter current at predetermined intervals correspondingto the sampling rate in the data acquisition units 14 and to store thetransmitter current samples at the transmitter control unit 16. This maybe facilitated in any suitable way, for example by disposing a shuntresistor in series with the transmitter current.

The data acquisition units 14 are each arranged to generate timereference data useable to control the times at which samples of thesurvey data are taken. Likewise, the transmitter control unit 16 isarranged to generate time reference data useable to control the times atwhich samples of the transmitter current are taken.

In this example, the transmitter control unit 16 and each dataacquisition unit 14 include the same components and, as a consequence,the data acquisition units 14 are able to function as a transmittercontrol unit 16 and vice versa. For ease of reference, in the followingdescription of embodiments of the invention, the data acquisition units14 and the transmitter control unit 16 will be referred to as “nodes”.

However, notwithstanding that the data acquisition units 14 and thetransmitter control unit 16 in the following embodiments include thesame components, it will be understood that this is not necessarily thecase. As an alternative, the data acquisition units 14 and thetransmitter control unit 16 may be configured so as to be dedicated totheir respective tasks and, as a result, not interchangeable.

The structure of a node 14, 16 is shown in FIG. 3.

Each of the nodes 14, 16 includes circuitry 20 and a power source, inthis example in the form of a rechargeable battery 22. As analternative, power may be supplied from an external power source.

The circuitry 20 includes a processing and control unit 26 forprocessing survey data received from sensors connected in use to thenode when the node is used as a data acquisition unit 14, for processingtransmitter current waveform data received from the transmitter 18 whenthe node is used as a transmitter control unit 16, and to control andcoordinate operation of the node 14, 16. The circuitry 20 also includesan analogue interface unit 28 for interfacing between the processing andcontrol unit 26 and sensors 15 or a transmitter 18 connected in use tothe node 14, 16, and a circuit protection unit 30 for protecting theanalogue interface unit 28 from damage which may occur as a result oflarge voltage transients from the sensors 15.

It will be understood that the type of sensors used will depend on theparticular type of geophysical survey operation being carried out. Inthe present example, the survey is a TEM type survey and the sensors arecoil-type sensors.

The circuitry 20 also includes an input/output interface 36 arranged tofacilitate transfer of information between the node 14, 16 and aseparate computing device or between two nodes 14, 16. In this example,the input-output interface 36 includes a serial interface forfacilitating transfer of synchronisation signals to the nodes 14, 16 forthe purpose of maintaining synchronisation of sample times, and an infrared interface for facilitating transfer of geophysical survey data ortransmitter current data between the node 14, 16 and a separatecomputing device using infra red radiation. In this example, the infrared interface is an IrDA interface. The input/output interface may alsoinclude a network interface (not shown). The input/output interface mayas an alternative be of a type which utilises wireless protocols, suchas Bluetooth.

The circuitry 20 also includes a transmitter interface 38 for use whenthe node operates as a transmitter control unit 16. The transmitterinterface 38 serves to transfer control instructions to the transmitter18 and may also be used to transfer current waveform data from thetransmitter 18 to the processing and control unit 26 for sampling.

The circuitry 20 also includes an LCD display 40 for displaying to auser information indicative of the status of operation of the node 14,16, a user control panel 42 for facilitating direct entry of controlinstructions to the node 14, 16 by a user, and an LED display 44 whichserves to indicate to a user the status of the node 14, 16, whether thenode 14, 16 has a fault, whether the remaining power in the battery 22is low, and so on.

The processing and control unit 26 receives signals indicative of timingand location data from a GPS antenna 32 and generates time referencedata which governs the time at which samples of the survey data or thetransmitter current are taken.

The processing and control unit 26 includes a processor 46, and a timingunit 48 in operative communication with the GPS antenna 32 and arrangedto generate the time reference data using the signals received from theGPS antenna 32.

In the case of a data acquisition unit 14, the time reference data isused to control the times at which samples of gathered survey datareceived from the sensors 15 are taken. In the case of a transmittercontrol unit 16, the time reference data is used to control the times atwhich samples of the transmitter current are taken. By associating thetime reference data with the samples of the received survey data and thesamples of the transmitter current, the system 10 is able to accuratelysynchronise the transmitter current with the received survey data.

The processing and control unit 26 also includes a data storage device54 arranged to store survey data received from sensors 15 connected tothe data acquisition unit 14 or to store transmitter current datareceived from the transmitter 18, depending on whether the node is adata acquisition unit 14 or a transmitter control unit 16. The datastorage device 54 is also used to store programs for controllingoperation of the node 14, 16. In the present example, the data storagedevice 54 is in the form of a FLASH memory

The processor 46 is arranged to control and coordinate all operations inthe node 14, 16 in accordance with programs stored in the data storagedevice 54. It will be understood that the programs may be pre-stored onthe data storage device 54 prior to deployment of the nodes on-site, orthe programs may be transferred to the nodes as part of the deploymentprocess by connecting a computing device to the node 14, 16 using theinput-output interface 36 and transferring the programs to the node 14,16 for storage on the data storage device 54. Such programs may inaddition or alternatively be located on a separate computing device towhich the survey data is to be transferred for analysis.

The transferred programs are arranged to cause appropriate timereference data to be generated using signals received from the GPSantenna 32, and to associate the time reference data with the surveydata samples or with the transmitter current data samples depending onwhether the node operates as a data acquisition unit 14 or a transmittercontrol unit 16.

In the case of a data acquisition unit 14, the stored programs are alsoarranged to cause the processor 46 to process survey data received fromsensors 15 so as to generate processed survey data of reduced volume andreduced noise relative to the received survey data and which is in amore useful format. The processed survey data together with associatedtime reference data is stored on the data storage device 54. Processingfunctions may be carried out during and/or after data acquisition.

The stored programs may include programs arranged to detect the presenceof a transient interference event such as an atmospheric discharge(lightning) or a surge on a power transmission line. Software enables adecision to be made as to which data has been affected by the transientinterference and, for the data which has been affected, a best guess ofthe true data for the relevant sample period is generated to replace theaffected data. This is achieved by carrying out a plurality of datagathering operations in a survey with each data gathering operationinvolving activation of the transmitter current and gathering of datasamples on deactivation of the transmitter current, and comparing eachsample of a particular data gathering operation with a correspondingsample of a subsequent or previous data gathering operation, or with acorresponding average sample value of a plurality of correspondingsamples taken during a plurality of data gathering operations. Since itis expected that the corresponding samples should differ only slightlyfrom each other, if some of the sample values differ significantly froma previous, a subsequent or an average sample value, the software may bearranged to ignore the affected part of the survey data or to ignore alldata gathered during the particular data gathering operation duringwhich the affected survey data was present.

The stored programs may include a stacking program arranged to increasethe signal-to-noise ratio by carrying out selective tapered stacking asa method of averaging long series of raw data into smaller series inorder to reduce the effects of interference and to reduce the datavolume. Stacking is achieved by averaging a sample over a large numberof repeat samples, that is, over a large number of data gatheringoperations. Repetitive data is significantly enhanced at the expense ofnon-repetitive data. The stacking program may be arranged such that thecontribution to the stacked data by each element of the raw data variesdepending on an assessment made by algorithms in the programs as to thequality of the elements. For example, if one or more samples areaffected by a transient interference event, the identified samples maybe ignored by the stacking program.

The stored programs may also include a windowing program arranged toincrease the signal-to-noise ratio by averaging a number of consecutivesamples taken from each sensor during a data gathering operation so asto produce a single representative sample value for the consecutivesamples.

It will be understood that by carrying out selective tapered stackingand/or by averaging a number of consecutive samples, it is possible toreduce the noise associated with received survey data to levels of theorder of 1 nV/A or less.

The stored programs may also include programs arranged to generate andcontinuously update estimates of the incoming interference from allsources such as power transmission lines, BLF transmitters, atmosphericsources, and so on. If harmonic interference is still detectable afterstacking has taken place, the interference can be removed using adigital filter arranged to remove the affected parts of the spectrum andreplace the affected spectrum with interpolated error-free spectra.

The degree of incoming interference may be detected in various ways. Forexample, two data gathering operations may be carried out withtransmitter currents of opposite polarity and the responses addedtogether so as to produce a representation of the noise only.

Any harmonic interference which is present in the survey data is in mostcases relatively easy to detect as the frequency of the harmonicinterference is generally at a different frequency to the frequency usedfor the transmitter 18. For example, harmonic interference caused bypower lines is generally at 50 Hz and can therefore be removed from thesurvey data by any appropriate filtering technique, such as digitalfiltering.

The stored programs may also include a drift detector program arrangedto detect a drift in relative timing between the node and thetransmitter by cross-correlating a measurement at a site with a previousmeasurement obtained at the time of deployment of the node at the site.Using this detection, timing drift can be corrected.

The stored programs may also include programs arranged to cause theprocessor 46 to convert waveforms corresponding to the received surveydata into the frequency domain using Fourier transform analysis, and toconvert data into meaningful units.

The stored programs may also include programs arranged to carry outdeconvolutions in order to remove the effects of various phenomena whichmay occur during the survey, such as variations in transmitter waveformsand sensor properties for example caused by a gradual decrease in powersupplied by the battery 22.

The stored programs may also include programs arranged to calculatequality control indicators for use in assessing the quality of thereceived survey data. For example, the programs may be arranged tocalculate standard deviation values for the survey data and, using noiseindicators for example derived from the above described estimate of thenoise present, to make a determination as to whether and/or which noisereduction process is necessary.

The stored programs may also enable the processor 46 to make decisionsin isolation concerning parameters associated with survey dataacquisition and processing. For example, the programs may enable theprocessor 46 to make decisions on the level of gain to apply to receivedsurvey data by analysing the magnitude of the survey data samples andadjusting the level of gain applied to the samples so as to preventsaturation of the amplifiers 60 during use.

Each of the nodes includes a multi-tasking operating system such asLinux which enables the node to carry out several functionssimultaneously. While acquiring data, the node can be interrogated, forexample using a hand-held computing device, in order for an operator tocarry out quality control of the performance of the node. Interrogationof the nodes can be carried out without interrupting the acquisition andprocessing of data being performed by the nodes. Operators are able todownload from the node any data stored in the data storage device 54,including information indicative of the quality of any data stored inthe data storage device 54. In this example, transfer of data between anode and the hand-held computing device takes place via the infra redinterface provided on the node and a corresponding infra red interfaceprovided on the hand-held computing device, although it will beunderstood that the transfer may take place in any other suitable way,for example via the serial interface or via a network interface.

The computing device to which data is transferred from the nodes mayinclude stored programs arranged to carry out processing operations onthe survey data received from the nodes. Since fixed loop EM data can betreated like geomagnetic or gravity data in that it can be representedby potential field equations, it is possible to extrapolate the data soas to provide estimates of the magnetic field above or below the surfaceof the survey area. For example, it is possible to interpolate themagnetic field so as to enhance detail of target elements located belowthe surface of the survey area. The interpolation process uses spatialderivatives derived between sensors disposed on the surface of thesurvey area to derive spatial derivatives in a vertical direction.

The stored programs are also arranged to process the received surveyeddata samples from the sensors so as to reduce noise of the type causedby atmospheric discharges and telluric currents. Such noise is generallyrelatively constant over a desired survey area.

In order to separate such noise from the survey data, the storedprograms are arranged to calculate the difference between simultaneoussamples from adjacent sensors and to divide each difference value by thedistance between the relevant sensors. This is equivalent to taking aspatial derivative over the survey data.

A representation of an array of data acquisition units 14 disposed on asurvey site is shown in FIG. 4.

It will be understood that calculations of spatial derivatives may betaken between sensors 15 disposed along a line in a first direction, forexample between first and second sensors 15 a and 15 b respectively,between sensors 15 disposed along a second line orthogonal to the firstline, such as between first and third sensors 15 a and 15 crespectively, and/or between sensors disposed generally diagonallyrelative to each other, such as between first and fourth sensors 15 aand 15 d respectively. The important aspect is that for each sensor 15spatial derivatives may be taken between the sensor 15 and any number ofadjacent sensors 15.

It will be understood that by calculating spatial derivatives in thisway, it is possible to derive processed survey data which is virtuallyfree of noise caused by atmospheric discharges, telluric currents, andthe like.

In some situations, it is not practically possible to calculatederivatives simultaneously over an entire survey area because a largenumber of sensors 15 and associated data acquisition units 14 would berequired and an insufficient number of data acquisition units 14 andsensors 15 are available. With such a situation, operators are requiredto move the available array of data acquisition units and sensorsseveral times in order to cover the entire survey area. However, sinceall proposed locations in the survey area for the sensors 15 are notsimultaneously occupied by the sensors 15, it is not possible todirectly calculate all possible spatial derivatives for each sensorlocation instantaneously.

In order to obtain spatial derivatives for all sensor locations, areference data acquisition unit and associated reference sensor 15 r maybe used, as shown in FIGS. 5 a and 5 b. In the present example, thereference sensor 15 r is disposed generally centrally of the proposedsurvey area, although this is not necessarily the case.

As indicated in FIG. 5 a, in order to cover an entire survey area, afirst line 55 of data acquisition units 14 and associated sensors 15 arefirst disposed in the survey area. Instantaneous spatial derivatives arethen taken simultaneously between adjacent nodes in the first line 55,such as between first and second sensors 15 a and 15 b respectively,between second and third sensors 15 b and 15 c respectively and betweeneach of the sensors in the first line 55 and the reference sensor 15 r.The sensors 15 in the first line 55 are then moved so as to be disposedin a second line 57, and simultaneous spatial derivatives betweenadjacent sensors 15 in the second line 57 and between each sensor 15 inthe second line 57 and the reference sensor 15 r are taken. In order tocalculate the instantaneous spatial derivative between a sensor 15 whenplaced in the first line 55 and an adjacent sensor 15 when placed in thesecond line 57, the spatial derivative calculated relative to thereference sensor 14 r when a sensor is in the second line 57 issubtracted from the spatial derivative calculated relative to thereference sensor 14 r when the sensor is in the first line 55. Thisprovides a spatial derivative corresponding to a simultaneous spatialderivative calculated between a sensor disposed in the first line 55 andan adjacent sensor disposed in the second line 57.

Since all the spatial derivatives for the whole survey are not actuallyderived from survey data gathered simultaneously, it may be necessary tocorrect the survey data for any variations which may occur as a resultof variations in transmitter current which may occur between differentsensor locations. The reference data acquisition unit 14 may be used tofacilitate correction of variations in the survey data by continuouslymeasuring the survey data obtained by the reference node during thewhole survey.

It will be understood that in order to correct for small variations insensitivity of the sensors 15 and the data acquisition units 14 prior tocarrying out a survey, the sensors 15 and data acquisition units 14should be calibrated in order to correct for variations caused bydifferences in orientations of the sensors and sensitivity of thesensors and data acquisition units.

In the present example, this is achieved by collecting survey data inresponse to a distant signal source that is only slowly spatiallyvarying over the survey area or not varying at all. For example, noisecaused by distant atmospheric discharges may be used as the signalsource and survey data collected in the absence of a transmittercurrent. Given the relatively small spatial separation of sensors in thesurvey area, it can be expected that the response at each sensor will bewell correlated between all sensors. For a given survey area, acorrection co-efficient may be associated with each sensor or, in thecase of multiple component sensors, a vector or tensor may be associatedwith each sensor.

The stored programs may also be arranged to integrate the survey data soas to achieve potentially cleaner conventional data. An integrationconstant may be added which may be derived using the reference dataacquisition unit and associated reference sensor.

The analogue interface unit 28, shown more particularly in FIG. 6,includes circuitry for four different signal channels, each channelincluding an amplifier 60 which receives survey data from a sensor 15 orfrom a transmitter 18 depending on whether the node is a dataacquisition unit 14 or a transmitter control unit 16. The filtered datais then passed to a low-pass filter 62 and an A/D converter 64, in thisexample a 24-bit converter.

Control of the amplifiers 60, filters 62 and the A/D converters 64including clocking of the A/D converters 64 and thereby sampling of thereceived survey data or the transmitter current, is carried out by theprocessing and control unit 26.

The protection unit 30 includes a separate protection circuit for eachchannel, each protection circuit serving to protect the circuitry of theanalogue interface unit 28 from damage due to large voltage transientswhich may be present on the signals input to the protection unit 30.

The nodes 14, 16 may also be provided with an accurate oscillator, inthis example an oven-controlled crystal oscillator (OCXO), useable bythe timing unit 48 to generate time reference data when a GPS signal isunavailable. The crystal oscillator produces an accurate frequencysignal which is used by the timing unit 48 to generate time referencedata. In this example, the time reference data is the output of acounter arranged to count the number of cycles of the signal produced bythe crystal oscillator. However, with this arrangement, since each node14, 16 includes a separate crystal oscillator, if a GPS signal isunavailable it is necessary to periodically synchronise the crystaloscillators with each other during the course of a survey. In practice,this is achieved by providing the transmitter control unit 16 with ahigh precision crystal oscillator, by providing each data acquisitionunit 14 with a precision crystal oscillator, and by periodicallyconnecting each data acquisition unit 14 with the transmitter controlunit 16 through the input/output interface 36 so as to compare thefrequency of the high precision crystal oscillator with the frequency ofthe precision oscillator and to compare the phasing of the counter inthe data acquisition unit with the phasing of the counter in thetransmitter control unit 16. Any discrepancy between the frequencies andcounters is removed by adjusting the frequency of the precision crystaloscillator and by adjusting the phasing of the counter associated withthe precision crystal oscillator.

When a GPS signal is unavailable for an extended period of time of theorder of several hours or more, there is a possibility that the timereference data derived from the oscillator will drift. During a surveywith an active source, the drift of a node's time reference datarelative to the source primary field waveform can be monitored by a nodewhilst it is positioned at a particular location. The drift iscalculated by cross-correlating a measurement at a particular time witha measurement taken at the time of deployment of the node at thelocation. Since it can be assumed that the drift is caused by a slowloss of synchronisation at the node, the drift can be corrected bymodifying the frequency of the oscillator and phasing of the counterassociated with the oscillator in accordance with the detected drift.

An example of a geophysical transient electromagnetic (TEM) survey usingthe above data acquisition system will now be described.

Operators first deploy one or more transmitter loops 12 at a suitablelocation for energising a desired survey area 80, and connect thetransmitter 18 to a transmitter loop 12. A transmitter control unit 16is connected to the transmitter 18 in order to control the transmitter18 and to sample the current flowing through the transmitter loop 12.

Operators then distribute data acquisition units 14 around the desiredsurvey area and connect each data acquisition unit 14 to one or moresensors 15, in this example coil-type sensors, by connecting the sensors15 to the inputs of the protection unit 30.

When deployed, the data acquisition units 14 and the transmitter controlunit 16 are switched on and programs residing in the data storagedevices 54 of the data acquisition unit 14 cause the data acquisitionunits 14 to commence retrieving signals from the sensors 15, takesamples of the signals, process the sampled signals, and record theprocessed signals. Similarly, the programs residing in the data storagedevice 54 of the transmitter control unit 18 cause the transmittercontrol unit 18 to control the transmitter 18, to commence retrievingsignals from the transmitter, take samples of the retrieved transmittersignals, process the sampled signals, and record the processed signals.If necessary, the operators provide information to the data acquisitionunit 14 and the transmitter control unit 16 to update the configurationof the units 14, 16 for the particular survey and the particular tasksto be carried out. In practice, the majority of settings for all unitsin a survey will be the same. The instructions given to update theconfiguration of the units 14, 16 include settings of the transmitterfrequency, the rate at which processed survey data is to be stored inthe data storage device 54, and other settings related to the processingof survey data. Instructions transferred to the units 14, 16 by anoperator are transmitted via the input/output interface 36 using aportable computing device.

Using the same portable computing device and the input/output interface36 of a data acquisition unit 14, an operator can view survey data fromthe data acquisition unit to verify its operation. Additionally,information provided to the operator via the LCD display 40 and the LEDdisplay 44 allows the operator to make a rapid assessment of thefunctioning of the data acquisition unit 14.

In the present example, the survey is a TEM type survey and thetransmitter 18 and associated transmitter loops 12 are controlled by thetransmitter control unit 16 so as to generate a magnetic field whichdecays over time. As a consequence, the samples of the survey datarecorded by the sensors 15 reduce in magnitude with each successivesample.

When a transmitter loop 12 is operational, a primary magnetic fieldwhich decays over time is generated which passes through the surveyarea, including through the prospective sub-surface volume. Electricallyconductive elements of the sub-surface volume respond to the primaryfields by conducting electric currents. These currents flowing in thesub-surface themselves generate secondary electromagnetic fields thatcan be diagnostic of the geology of the sub-surface volume. Coil sensorsdisposed in the vicinity of the sub-surface volume detect the primaryand secondary fields and generate survey data in the form of a voltagewhich reduces in magnitude over time, the voltage being fed to theanalogue interface unit 28 via the protection unit 30 of a dataacquisition unit 14. At the analogue interface unit 28, the voltage isamplified, filtered, converted to digital, and sampled using the timereference data. The processor 46 then processes the sampled survey datain accordance with the processing steps described above in order toincrease signal-to-noise ratio and reduce the volume of data.

At pre-determined intervals governed by the programs and settings storedin the data storage device 54, processed survey data is stored in thedata storage device 54 of each data acquisition unit 14. In addition tothe survey data itself, time reference data indicative of the timing ofsamples, information indicative of the location of sensors, and anyother information that is needed for processing of the survey data isrecorded in the data storage device 54.

During the course of the survey, an operator visits each dataacquisition unit 14 for the purpose of confirming correct operation ofthe units 14, 16. At this time, processed data or other forms of datamay be downloaded from the data acquisition units via the input/outputinterface 36 to a portable computing device carried by the operator forthe purpose of analysing data quality and collating data from thesurvey.

A survey may include one or more data gathering operations, that is, oneor more operations involving activation of the transmitter, deactivationof the transmitter and gathering of data samples as necessary.

When the survey has been completed, all survey data representative ofsensor responses and the current flowing through the transmitter loop 12is transferred to a portable computing device from the units 14, 16. Itwill be appreciated that since the received survey data is processed bythe processor 46 so as to reduce the volume of survey data, only arelatively small and inexpensive data storage device is required in eachdata acquisition unit 14 and only a relatively short time is required tocollect and collate data from all data acquisition units. The portablecomputing device then processes the survey data so as to generatespatial derivatives and so as to carry out any other desired processingactions.

It will also be appreciated that since the received survey data isstored at the nodes 14, 16 for subsequent downloading and analysis aftercompletion of the survey, and since the nodes 14, 16 generate timereference data for the received survey data either through GPS orthrough a local crystal oscillator, the nodes are effectivelyautonomous, and cumbersome and expensive cabling between the nodes 14and the central computing device and/or a timing device is notnecessary.

A data acquisition system 70 in accordance with an alternativeembodiment of the present invention is shown in FIG. 6

The alternative system 70 is suitable for use in areas where GPS in notavailable. Like features are indicated with like reference numerals.

The alternative system 70 includes a roving node 74 which serves tomaintain synchronisation between the received survey data and thetransmitter 18.

The transmitter control unit 16 and the roving node 74 include a highprecision oven-controlled crystal oscillator and each of the dataacquisition units 14 include a less expensive precision oven-controlledcrystal oscillator.

In operation, an operator periodically connects the roving node 74 toeach data acquisition unit 14 via the input/output interface 36 so as tosynchronise the precision crystal oscillators in the data acquisitionunits 14 with the high precision crystal oscillators in the roving node74.

With this embodiment, instead of the data acquisition units 14 receivinginstructions from a portable computing device, the data acquisitionunits 14 may receive instructions from the roving node 74 when theroving node 74 is connected to the data acquisition units 14.

Modifications and variations as would be apparent to a skilled addresseeare deemed to be within the scope of the present invention.

1-62. (canceled)
 63. A data acquisition system for gathering geophysical data, said system comprising: at least one data acquisition unit connectable to a plurality of sensors and being arranged, during use, to simultaneously gather geophysical data from the sensors, the at least one data acquisition unit comprising time referencing means arranged to generate time reference data usable to control the time at which samples of geophysical data are taken; and means for calculating spatial derivatives between simultaneous samples associated with adjacent sensors connected, during use, to the at least one data acquisition unit.
 64. The data acquisition system as claimed in claim 63, wherein the time referencing means comprises a GPS receiver.
 65. The data acquisition system as claimed in claim 63, wherein the time referencing means comprises an accurate oscillator.
 66. The data acquisition system as claimed in claim 65, wherein the accurate oscillator comprises a precision oven controlled crystal oscillator, and the time referencing means further comprises a counter arranged to count signals generated by the crystal oscillator.
 67. The data acquisition system as claimed in claim 65, wherein the data acquisition unit is arranged to receive synchronisation signals useable to adjust a frequency of the oscillator and thereby adjust the times at which samples of geophysical data are taken so that the times at which samples of geophysical data are taken are synchronised with the times at which samples of geophysical data are taken in other data acquisition units.
 68. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to receive and store programs for subsequent execution.
 69. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to calculate an average sample value for a plurality of corresponding repeat sample values when a plurality of data gathering operations are carried out as part of a geophysical survey so as to reduce an effect of interference on the samples and reduce the quantity of data.
 70. The data acquisition system as claimed in claim 69, wherein the data acquisition unit is arranged to compare repeat samples and to discard samples which differ by a predetermined amount from a majority of the repeat samples.
 71. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to calculate an average sample value for a plurality of consecutive samples taken during a data gathering operation carried out as part of a geophysical survey so as to produce a representative sample for the consecutive samples.
 72. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to estimate the amount of interference present at a survey site.
 73. The data acquisition system as claimed in claim 72, wherein the amount of interference present is estimated by carrying out a first data gathering operation with an incident magnetic field of a first polarity so as to produce a first response, carrying out a second data gathering operation with an incident magnetic field of a second polarity so as to produce a second response, and calculating a sum of the first and second responses so as to cause the first and second responses to cancel out.
 74. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to filter gathered geophysical data so as to remove periodic interference.
 75. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to convert gathered geophysical data into frequency domain using Fourier transform analysis.
 76. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to generate at least one quality control indicator for use in assessing a quality of the gathered geophysical survey data.
 77. The data acquisition system as claimed in claim 76, wherein the data acquisition unit is arranged to calculate a standard deviation value for the gathered geophysical survey data.
 78. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to adjust a level of gain applied to gathered geophysical survey data based on an assessment of a magnitude of the gathered geophysical survey data.
 79. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to downward extrapolate gathered geophysical survey data so as to enhance detail of a target located below a surface of a survey area.
 80. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is connectable to an energy source, the data acquisition unit is arranged to gather energy source output data from the energy source, and the time referencing means is arranged so as to sample the gathered energy source output data.
 81. The data acquisition system as claimed in claim 80, wherein the system is arranged to correct for variations in magnitude of the energy source output during a geophysical survey.
 82. The data acquisition system as claimed in claim 76, wherein the system is arranged to correct for a variation in magnitude of the gathered geophysical data caused by a variation in power supplied to the energy source.
 83. The data acquisition system as claimed in claim 63, further comprising at least one interface arranged to facilitate transfer of geophysical data and/or programs to or from the data acquisition unit.
 84. The data acquisition system as claimed in claim 83, wherein the data acquisition unit comprises a multi-tasking operating system.
 85. The data acquisition system as claimed in claim 84, wherein the data acquisition unit is arranged to facilitate transfer of geophysical data from the data acquisition unit during a geophysical survey.
 86. The data acquisition system as claimed in claim 83, wherein the interface comprises at least one of an infra red interface, a serial interface, and a network interface.
 87. The data acquisition system as claimed in claim 63, wherein the data acquisition unit is arranged to store a correction coefficient for each sensor connected during use to the data acquisition unit, each correction coefficient being used to correct for variations in sensor sensitivity.
 88. The data acquisition system as claimed in claim 63, further comprising display means arranged to provide information indicative of operation of the data acquisition unit to an operator.
 89. The data acquisition system as claimed in claim 63, wherein the data acquisition unit includes the means for calculating spatial derivatives.
 90. The data acquisition system as claimed in claim 63, wherein the means for calculating spatial derivatives is separate from the data acquisition unit.
 91. The data acquisition system as claimed in claim 90, further including a portable computing device, the portable computing device including the means for calculating spatial derivatives.
 92. The data acquisition system as claimed in claim 63, comprising a plurality of data acquisition units.
 93. The data acquisition system as claimed in claim 63, further comprising: at least one reference data acquisition unit, each reference data acquisition unit being connectable to at least one reference sensor and being arranged, during use, to gather geophysical data from the at least one reference sensor, and to take samples of the geophysical data gathered from the at least one reference sensor; wherein the means for calculating spatial derivatives between samples associated with adjacent sensors is arranged to calculate first spatial derivatives between at least some of the sensors and the at least one reference sensor connected to the reference data acquisition unit during a first data gathering operation when the sensors are disposed in a first location, to calculate second spatial derivatives between at least some of the sensors and the at least one reference sensor connected to the reference data acquisition unit during a second data gathering operation when the sensors are disposed in a second location, and to calculate a difference spatial derivative between the first and second spatial derivatives, each said difference spatial derivative being indicative of a spatial derivative between a sensor disposed in a first location and a sensor disposed in a second location.
 94. The data acquisition system as claimed in claim 93, further comprising means for calculating an integral of the spatial derivatives.
 95. The data acquisition system as claimed in claim 93, further comprising an energy source arranged to generate and direct energy towards a sub-surface volume so as to cause a geophysical response and thereby cause generation of the geophysical signals.
 96. The data acquisition system as claimed in claim 95, wherein the energy source includes a transmitter and a transmitter loop.
 97. The data acquisition system as claimed in claim 93, further comprising an energy source control unit connectable to the energy source and arranged to gather output data from the energy source, the energy source control unit comprising time referencing means arranged to generate time reference data usable to control the time at which samples of the energy source output data are taken and to associate the energy source output data with the time reference data.
 98. The data acquisition system as claimed in claim 97, wherein the energy source control unit is a transmitter control unit arranged to control a transmitter so as to energise a transmitter loop in accordance with a predetermined frequency.
 99. The data acquisition system as claimed in claim 97, wherein the energy source control unit includes the same components as the data acquisition unit so that the transmitter control unit is capable of carrying out the functions of the data acquisition unit and vice versa.
 100. A method of acquiring geophysical data, said method including the steps of: providing at least one data acquisition unit arranged to simultaneously gather geophysical data from a plurality of sensors connected in use to the at least one data acquisition unit; connecting a plurality of sensors to the at least one data acquisition unit; generating at the data acquisition unit time reference data usable to control the time at which gathering of samples of geophysical data are taken; and calculating spatial derivatives between simultaneous samples associated with adjacent sensors connected during use to the at least one data acquisition unit.
 101. The method of acquiring geophysical data as claimed in claim 100, further comprising the steps of: providing at least one reference data acquisition unit arranged, during use, to gather geophysical data from at least one reference sensor; connecting each of the at least one reference data acquisition unit to at least one of the at least one reference sensors; calculating first spatial derivatives between at least some of the sensors connected to the data acquisition units and the at least one reference sensor connected to the at least one reference data acquisition unit during a first data gathering operation when the sensors are disposed in a first location; calculating second spatial derivatives between at least some of the sensors connected to the data acquisition units and the at least one reference sensor connected to the at least one reference data acquisition unit during a second data gathering operation when the sensors are disposed in a second location; and calculating a difference spatial derivative between the first and second spatial derivatives, each said difference spatial derivative being indicative of a spatial derivative between the first location and a sensor the second location.
 102. The method as claimed in claim 100, further comprising means for calculating an integral of the spatial derivatives.
 103. The method as claimed in claim 100, wherein the step of generating time reference data comprises the step of providing a GPS receiver.
 104. The method as claimed in claim 100, wherein the step of generating time reference data comprises the step of providing an oscillator.
 105. The method as claimed in claim 104, wherein the oscillator comprises a precision oven controlled crystal oscillator, and the step of generating time reference data further comprises the step of providing a counter arranged to count signals generated by the crystal oscillator.
 106. The method as claimed in claim 104, further comprising the step of facilitating reception at the data acquisition unit of synchronisation signals useable by the processing means to adjust a frequency of the oscillator and thereby adjust the time at which samples of geophysical data are taken so as to synchronise the time at which samples of geophysical data are taken with the time at which samples of geophysical data are taken in other data acquisition units.
 107. The method as claimed in claim 100, further comprising the steps of receiving and storing programs at the data acquisition unit for subsequent execution by the processing means.
 108. The method as claimed in claim 100, further comprising the step of calculating an average sample value for a plurality of corresponding repeat sample values when a plurality of data gathering operations are carried out as part of a geophysical survey so as to reduce an effect of interference on the samples and reduce the quantity of data.
 109. The method as claimed in claim 100, further comprising the step of comparing repeat sample values and discarding samples which differ by a predetermined amount from a majority of the repeat sample values.
 110. The method as claimed in claim 100, further comprising the step of calculating an average sample value for a plurality of consecutive samples taken during a data gathering operation carried out as part of a geophysical survey so as to produce a representative sample for the consecutive samples.
 111. The method as claimed in claim 100, further comprising the step of estimating the amount of interference present at a survey site.
 112. The method as claimed in claim 111, wherein the amount of interference present is estimated by carrying out a first data gathering operation with an incident magnetic field of a first polarity so as to produce a first response, carrying out a second data gathering operation with an incident magnetic field of a second polarity so as to produce a second response, and calculating the sum of the first and second responses so as to cause the first and second responses to cancel out.
 113. The method as claimed in claim 100, further comprising the step of filtering gathered geophysical data so as to remove periodic interference.
 114. The method as claimed in claim 100, further comprising the step of converting gathered geophysical data into frequency domain using Fourier transform analysis.
 115. The method as claimed in claim 100, further comprising the step of correcting for variations in magnitude of an energy source during a geophysical survey.
 116. The method as claimed in claim 115, wherein the step of correcting for variations in magnitude includes the step of correcting for a variation in magnitude of the energy source caused by a variation in power supplied to the energy source.
 117. The method as claimed in claim 100, further comprising the step of generating at least one quality control indicator for use in assessing the quality of the gathered geophysical survey data.
 118. The method as claimed in claim 117, further comprising the step of calculating a standard deviation value for the gathered geophysical survey data.
 119. The method as claimed in claim 100, further comprising the step of adjusting a level of gain applied to gathered geophysical survey data based on an assessment of the magnitude of the gathered geophysical survey data.
 120. The method as claimed in claim 100, further comprising the step of downward extrapolating gathered geophysical survey data so as to enhance detail of a target located below a surface of a survey area.
 121. The method as claimed in claim 100, wherein the method further comprises the step of facilitating transfer of processed geophysical data and/or programs to or from the data acquisition unit.
 122. The method as claimed in claim 100, further comprising the step of providing each data acquisition unit with display means for providing information indicative of operation of the data acquisition unit to an operator.
 123. The system as claimed in claim 95, wherein the system is arranged to correct variations in the energy source using the reference data acquisition unit and associated reference sensor.
 124. The method as claimed in claim 115, further comprising the step of correcting variations in the energy source using the reference data acquisition unit and associated reference sensor.
 125. A data acquisition system for gathering geophysical data, said system comprising: at least one data acquisition unit connectable to a plurality of sensors and being arranged, during use, to simultaneously gather geophysical data from the sensors, the at least one data acquisition unit comprising time referencing means arranged to generate time reference data usable to control a time at which samples of geophysical data are taken; and a processor arranged to calculate spatial derivatives between simultaneous samples associated with adjacent sensors connected during use to the at least one data acquisition unit. 